Hybrid micro electronic assemblies have employed laser trimmable components such as thin and thick film resistors, capacitors, etc. Such components are applied at a top surface of a substrate, so that a laser (or other means including mechanical means) can make a cut on the trimmable component to change its value. The trimming generally occurs by using the laser to remove part of the material that forms said component. This provides a fine-tuned performance. When the materials to be trimmed are non metallic, lower power is required than when the feature to be trimmed is metallic. If the substrate is ceramic, then damage to surrounding or underlying material is of somewhat less concern.
However, for trim actions performed on an organic-type substrate materials, severing of metallic conductors such as traces formed of copper foil (where greater laser power is required) can damage adjacent or underlying components. Such trimmable components are generally applied to a substrate that is robust enough to avoid damage by the laser beam (e.g. ceramic material). Ceramic substrates provide adequate resistance to damage, but are costly compared to other substrate materials.
Low-cost substrate materials include epoxy resin laminates such as FR-4 and other non-ceramic plastic and resin materials including Kapton,Teflon and other organic based materials. These materials are substantially lower in cost than ceramic materials. However, they are unsuited for severing metallic conductors by laser action for the following reason. A typical FR-4 circuit board having an embedded ground plane can be damaged during the laser trimming of a surface component, because the beam can bum partially into the surface of the board, and penetrate even to the level of lower conductor layers. This burning of the circuit board material generates partially conductive carbon material, which can create a resistive path of uncertain conductivity from the surface component to the ground plane. In other applications, the beam may drill deep enough to damage other embedded (or opposite side) components. Such an effect on sensitive circuits such as a high impedance attenuator, renders the affordable organic materials unsuitable. Accordingly, there is a need for a low-cost methodology to enable the use of laser-trimmable components. The preferred embodiment provides this in the following: